North American Network Operators Group
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Re: Type of Service (TOS)

  • From: Iljitsch van Beijnum
  • Date: Mon May 10 17:59:58 2004 
On 10-mei-04, at 18:57, Vicky Rode wrote:
Say if I had a qos appliance installed on networks between a lan and a wan box would the qos policies be carried across wan end points (point to point connection)? In other words, will the router retain the TOS bits across to the other side of the wan connection to provide QoS-style priority for the packets or will it clear the TOS bits?
First of all, most routers that apply quality of service mechanisms don't set the type of service in the process. However, many can use the type of service as input for their QoS decisions.

Second, routers generally pass these bits unmodified, unless specifically instructed to change them.

Third, these bits are often interpreted as a diffserv code point rather than the original type of service meaning.

One setup I've used myself is set the type of service selectively using a rate limit on a Cisco router, and then do traffic shaping for packets matching this type of service on another router. AFAIK this isn't a particular common setup, though.